Spectroscopy of single light-harvesting complexes from purple photosynthetic bacteria at 1.2 K

In this Letter we present the first observation of the fluorescence-excitat ion spectra of individual light-harvesting complexes (LH2) from purple phot osynthetic bacteria at 1.2 K. The spectra reveal the electronic transitions to the individual excitonic states of the assembly of absorbing bacterioch lorophyll a (BChl a) molecules. From the experiment we can extract informat ion about site energies of the individual pigments, degree of disorder, dip olar coupling, excited-state dynamics, and spectral diffusion.